A number of ocular diseases that result in vision loss are associated with changes in the retinal vasculature. Traditionally, fluorescein and/or indocyanine green angiography have been used to assess these changes, but objective quantification can be challenging with these methods due to dye leakage and/or staining. Optical coherence tomography (OCT) is a noninvasive, depth resolved, volumetric imaging technique that uses principles of interferometry to provide cross-sectional and three-dimensional (3D) imaging of biological tissues. OCT has become part of the standard of care in ophthalmology and is commonly used to visualize retinal morphology. In recent years OCT methods have been extended to allow visualization of blood flow within tissues—an emerging technology termed “OCT angiography.” Because OCT angiography does not require the use of injectable dyes, it is not affected by leakage and staining issues and is, thus, more amenable to quantification than dye-based approaches. OCT angiography utilizes variation in the OCT signal on consecutive cross-sectional B-scans at the same location to contrast flowing red blood cells in the vessel lumen from surrounding structural tissue. Because OCT angiography has consistently high contrast for capillary details and is not affected by leakage and staining, quantification is more straightforward than with dye injection methods. By quantifying OCT signal variation between B-scans, for example by calculating decorrelation or speckle variance between images, it is possible to discriminate regions of blood flow (i.e., retinal vasculature) from static tissue and thereby quantify vascular characteristics such as vessel density, vessel area, and avascular area. An efficient OCT angiography algorithm called split-spectrum amplitude-decorrelation angiography (SSADA) has been used in a commercial system to visualize and quantify changes in the vascular networks of the eye.
OCT angiography data is often presented as a projection of the three dimensional dataset onto a single planar image called a 2D en face angiogram. Construction of such an en face angiogram requires the specification of the upper and lower depth extents that enclose the region of interest within the retina to be projected onto the planar image. Once generated, the en face angiogram image may be used to quantify various features of the retinal vasculature, for example, vessel density. This quantification typically involves the setting of a threshold value on the en face angiogram to separate real flow signal in blood vessels from noise, which can arise from bulk tissue motion or from within the OCT system itself. On macular angiograms, the threshold can be based on the average flow signal at a noise region, such as the foveal avascular zone (FAZ), which is known to be free of blood vessels in healthy eyes.